This invention relates to metals such as magnesium (Mg), cadmium (Cd), antimony (Sb), zinc (Zn) and tellurium (Te) that have purities of about 99.9999 wt % (6N) and above which have been obtained by heating feed metals and distilling them for purification. The invention also relates to a method and an apparatus for producing such high-purity metals.
In the manufacture of semiconductor devices which are seeing increasing demand as the result of the recent sophistication of electronics and declining cost, the need to use feed metals of higher purity is ever increasing. The fabrication of semiconductor devices such as blue-light laser diodes presents a demand for high-purity magnesium. In particular, the development of double heterostructure blue-laser diode devices is highly dependent on the quality of the material used in the cladding layer. Metals of high purity such as high-purity magnesium (Mg) generally contain sulfur (S), sodium (Na), aluminum (Al), silicon (Si), potassium (K), calcium (Ca), chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni), copper (Cu), arsenic (AS), antimony (Sb), lead (Pb), fluorine (F), phosphorus (P), chlorine (Cl), silver (Ag), bismuth (Bi), gallium (Ga), lithium (Li), molybdenum (Mo), titanium (Ti) and boron (B) (these elements contained in Mg are collectively referred to as impurities and the sum of their contents is referred to as the total impurity content; in the case where high-purity Mg is used as a semiconductor material, the inclusion of up to 100 ppm of zinc need not be particularly avoided and presents no problem in use; hence, a zinc content of up to 100 ppm is not dealt with as an impurity). The impurities in the high-purity magnesium used in the cladding layer of the double heterostructure blue-laser diode and for other purposes are by no means desirable for the performance of semiconductor lasers and this is another reason for the increasing need to produce magnesium and other metals of ultra-high purity. Magnesium and zinc are metals having comparatively high vapor pressures and more difficult to purify than other semiconductor materials by distillation.
In the conventional process of producing high-purity metals by purification through distillation of metals such as magnesium, the metal vapor generated by heating in a high-vacuum atmosphere is recovered by allowing it to solidify on cooling plates in the passageway of vapors. For example, International Patent Publication No. 502565/1999 describes a technique in which a plurality of baffle plates are provided over three zones in a passageway for the magnesium vapor generated by heating a magnesium feed within a crucible in a high-vacuum atmosphere and the magnesium vapor is cooled with the temperature of the baffle plates being controlled to decrease gradually toward the higher position, thereby utilizing the difference between the solidification temperatures of impurities in the magnesium vapor such that high-purity magnesium is fractionally solidified in a specified zone in the intermediate section.
However, it is difficult on an industrial scale to ensure that only the desired high-purity metal such as magnesium is efficiently cooled and recovered from the metal vapor in the passageway of vapors. If the separation of high-purity metal is to be achieved by the difference in solidification temperature, it is difficult to exclude the entrance of impurities having only a small difference in solidification temperature. In order to obtain the desired high-purity magnesium, the specified zone for recovery must maintain a very small temperature range but this only results in a very low yield. On the other hand, if one wants a higher yield, the purity of magnesium has to be lowered. If smaller cooling plates are used with a view to maintaining a smooth passage of vapors during recovery of the high-purity metal, the yield remain low and is within a limited range since the amount of recovery depends on the size of the cooling plates. If larger cooling-plates are used, the vapor passageway becomes so narrow as to prevent the passage of metal vapors, again causing the yield to remain low in a limited range.
An object, therefore, of the invention is to produce magnesium and other metals of high-purity from feed metals by purification through distillation.
Another object of the invention is to provide a purification process and apparatus by which the desired high-purity metal can be produced in high yield and efficiency at low cost.
The present inventors conducted intensive studies in order to solve the aforementioned problems of the prior art. As a result, they found that by condensing part of the vapor of a feed metal in a crucible instead of causing all vapor to solidify fractionally in the passageway of vapors, a molten condensate with an increased total content of impurities would be obtained and that by returning the melt into the crucible, the impurities are concentrated in the feed metal.
To attain its first to third objects, the present invention provides the following.
1. A high-purity metal-containing Cl, F and S in a respective amount of no more than 0.1 ppm, with the total impurity content being no more than 1 ppm.
2. The high-purity metal of item 1, in which said metal is magnesium or zinc.
3. A process for metal purification comprising a first step for heating a feed metal in a feed crucible 1 (the reference numeral is keyed to the accompanying drawing and this applies to the following description) to generate the vapor of said metal, a second step for directing said vapor into a condensation passageway for vapors, where part of the vapor is condensed to generate a molten condensate, and a third step for directing said vapor through said condensation passageway for vapors into a solidification crucible 2 so that it is cooled to get said metal in high-purity form to solidify from it.
4. The process of item 3, which further includes a fourth step for returning said molten condensate into said feed crucible 1.
5. An apparatus for metal purification comprising a vessel 3 for creating a vacuum atmosphere, a feed heating zone with an open top that contains a feed crucible 1 to be charged with a feed metal, a condensation zone on top of said feed heating zone in which condensation vapor passage plates 5 that are each convex downwardly and provided with a vapor passage hole 4 in a generally central area and condensation vapor passage plates 5 that are each convex upwardly and provided with a plurality of vapor passage holes 4 in the non-central area alternate with each other at given spacings and are stacked in general symmetry with respect to a plane, and a solidification zone on top of said condensation zone for solidifying said metal.
6. An apparatus for metal purification comprising a vessel 3 for creating a vacuum atmosphere, a feed heating zone with an open top that contains a feed crucible 1 to be charged with a feed metal to generate the vapor of said metal, a condensation zone with an open top and an open bottom communicating with the top of said feed heating zone and in which a plurality of condensation vapor passage plates 5 that form a condensation passageway for vapors in which said vapor is allowed to pass upwardly only through the vapor passage holes 4 made in said plates 5 are stacked vertically at given spacings, and a solidification zone with an open top and an open bottom communicating with the top of said condensation zone and which contains a solidification crucible 2 which is cooled externally so that said metal of high purity is solidified from said vapor after it has passed through said condensation zone, said plurality of condensation vapor passage plates 5 being such that condensation vapor passage plates 5 in the form of an inverted cone or dome which are each convex downwardly and provided with a vapor passage hole 4 in a generally central area and condensation vapor passage plates 5 in a conical or dome shape that are each convex upwardly and provided with a plurality of vapor passage holes 4 in the non-central area alternate with each other and are stacked in general symmetry with respect to a plane, and a heater 6 for heating said feed heating zone and said condensation zone being provided within or outside said vessel 3.
7. The apparatus of item 6, in which said vessel 3 for creating a vacuum atmosphere further contains an entrapment/solidification zone with an open top and an open bottom communicating with the top of said solidification zone and in which a plurality of entrapment/solidification vapor passage plates 7 that form an entrapment/solidification passageway for vapors which is cooled externally and in which said vapor after passing through said condensation zone is allowed to pass upwardly only through the vapor passage holes 4 made in said plates 7, thereby solidifying said vapor are stacked vertically at given spacings, said plurality of entrapment/solidification vapor passage plates 7 being such that entrapment/solidification vapor passage plates 7 in the form of an inverted cone or dome which are each convex downwardly and provided with a plurality of vapor passage hole 4 in the non-central area and entrapment/solidification vapor passage plates 7 in a conical or dome shape that are each convex upwardly and provided with a vapor passage hole 4 in a generally central area alternate with each other and are stacked in general symmetry with respect to a plane.